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class="breadcrumbs" data-v-06225672><li data-v-06225672><a href="/" title="首页" class="iconfont icon-home router-link-active" data-v-06225672></a></li> <li data-v-06225672><a href="/mianshi/#面试" data-v-06225672>面试</a></li><li data-v-06225672><a href="/mianshi/#前端面试" data-v-06225672>前端面试</a></li><li data-v-06225672><a href="/mianshi/#基础篇" data-v-06225672>基础篇</a></li></ul> <div class="info" data-v-06225672><div title="作者" class="author iconfont icon-touxiang" data-v-06225672><a href="https://github.com/linxin1123" target="_blank" title="作者" class="beLink" data-v-06225672>lingxin</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06225672><a href="javascript:;" data-v-06225672>2023-02-26</a></div> <!----></div></div></div> <!----> <div class="content-wrapper"><div class="right-menu-wrapper"><div class="right-menu-margin"><div class="right-menu-title">目录</div> <div class="right-menu-content"></div></div></div> <h1><img src="">JavaScript(五)<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="javascript-五"><a href="#javascript-五" class="header-anchor">#</a> JavaScript(五)</h1> <h2 id="题目目录"><a href="#题目目录" class="header-anchor">#</a> 题目目录</h2> <ol><li><a href="#q1">浏览器缓存</a></li> <li><a href="#q2">WebSocket</a></li> <li><a href="#q3">尽可能多的说出你对Electron的理解</a></li> <li><a href="#q4">深浅拷贝</a></li> <li><a href="#q5">防抖/节流</a></li> <li><a href="#q6">谈谈变量提升</a></li> <li><a href="#q7">什么是单线程,和异步的关系</a></li> <li><a href="#q9">前端面试之hybrid</a></li> <li><a href="#q10">前端面试之组件化</a></li> <li><a href="#q11">前端面试之MVVM浅析</a></li> <li><a href="#q12">实现效果，点击容器内的图标，图标边框变成border 1px soild red,点击空白处重置</a></li> <li><a href="#q13">请简单实现双向数据绑定mvvm</a></li> <li><a href="#q14">实现Storage,使得该对象为单例，并对localstorage进行封装设置值setItem(key,value)和getItem(key)</a></li> <li><a href="#q15">说说event loop</a></li> <li><a href="#q16">说说事件流</a></li> <li><a href="#q17">为什么canvas的图片为什么有跨域问题</a></li> <li><a href="#q18">我现在有一个canvas，上面随机布者一些黑块，请实现方法，计算canvas上有多少个黑块</a></li> <li><a href="#q19">手写实现一个promise</a></li> <li><a href="#q20">说说从输入URL到看到页面发生的全过程，越详细越好</a></li></ol> <h3 id="q1">1. 浏览器缓存</h3> <blockquote><p>浏览器缓存分为强缓存和协商缓存，当客户端请求某个资源时，获取缓存的流程如下</p></blockquote> <ul><li>先根据这个资源的一些http header 判断它是否命中强缓存，如果命中，则直接从本地获取缓存数据，不会发请求到服务器</li> <li>当强缓存没有命中时，客户端会发生请求到服务器，服务器通过另一些request header 验证这个资源是否命中协商缓存，称为http再验证，如果命中，服务器将请求返回，但不返回资源，而是告诉客户端直接从缓存中获取，客户端收到返回后就会从缓存中获取资源</li> <li>强缓存和协商缓存的共同之处在于：如果命中缓存，服务器都不会返回资源，区别是，强缓存不会发送请求到服务器，协商缓存会</li> <li>协商缓存也没命中，服务器就会将资源发送回客户端</li> <li>ctrl+f5 强制刷新页面时，直接从服务器加载，跳过强缓存和协商缓存</li> <li>f5刷新页面，跳过强缓存，检查协商缓存</li></ul> <h3 id="强缓存"><a href="#强缓存" class="header-anchor">#</a> 强缓存</h3> <ul><li>Expires (该字段是http1.0时的规范，值为一个绝对时间的CMT格式的时间字符串，代表缓存资源的过期时间)</li> <li>Cache-Control:max-age(该字段是http1.1的规范，强缓存利用其max-age值来判断缓存资源的最大生命周期，它的值单位为秒)</li></ul> <h4 id="协商缓存"><a href="#协商缓存" class="header-anchor">#</a> 协商缓存</h4> <ul><li>last-modified(值为资源最后更新时间，随服务器response返回)</li> <li>If-Modified-Since(通过比较两个时间来判断资源在两次请求期间是否有过修改，如果没有修改，则命中协商缓存)</li> <li>ETag(表示资源内容的唯一标识，随服务器response返回)</li> <li>If-None-Match(服务器通过比较请求头部的If-None-Match)与当前资源的ETag是否一致判断资源是否在两次请求之间有过修改，如果没有修改，则命中协商缓存</li></ul> <h3 id="q2">2. WebSocket</h3> <blockquote><p>由于http存在一个明显的缺陷（消息只能由客户端推送到服务器端，而服务器不能主动推送到客户端），导致如果服务器有连续的变化，这时只能使用轮询，而轮询效率过低，并不适合，webSocket被发明出来</p></blockquote> <blockquote><p>相较http具有以下优点</p></blockquote> <ul><li>支持双向通信，实时性更强</li> <li>可以发送文本，也可以二进制文件</li> <li>协议标识符是ws，加密后wss</li> <li>较少的控制开销，连接创建后，ws客户端，服务器进行数据交换时，协议控制的数据包头部较小，在不包含头部的情况下，服务器到客户端的包头只有2-10字节（取决于数据包长度），客户端到服务器端的话，需要加上额外的4字节的掩码，而HTTP协议每次通信需要携带完整的头部</li> <li>支持扩展。ws协议定义了扩展，用户可以扩展协议，或者实现自定义的子协议</li> <li>无跨域问题</li></ul> <blockquote><p>实现⽐较简单，服务端库如 socket.io 、 ws ，可以很好的帮助我们⼊⻔。
⽽客户端也只需要参照 api 实现即可</p></blockquote> <h3 id="q3">3. 尽可能多的说出你对Electron的理解</h3> <blockquote><p>最重要的一点，electron实际上是一个套了Chrome的nodejs程序</p></blockquote> <ul><li>chrome：无各种兼容性问题</li> <li>NodeJs：NodeJs能做的它也能做</li></ul> <h3 id="q4">4. 深浅拷贝</h3> <ul><li>浅拷贝
<ul><li>Object.assign</li> <li>或者展开运算符</li></ul></li> <li>深拷贝
<ul><li>JSON.parse(JSON.stringify(obj))
<ul><li>会忽略undefined</li> <li>不能序列化函数</li></ul></li></ul></li></ul> <h3 id="q5">5. 防抖/节流</h3> <blockquote><p>滚动事件需要做个复杂计算或者实现一个按钮的防二次点击操作</p></blockquote> <ul><li>防抖和节流的作用都是防止函数多次调用，区别在于，假设一个用户一直触发这个函数，且每次触发函数的间隔小于wait，防抖的情况下只会调用一次，而节流的情况会每隔一定时间（参数wait）调用函数</li></ul> <h4 id="防抖函数"><a href="#防抖函数" class="header-anchor">#</a> 防抖函数</h4> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">/**
* @param func  回调函数
* @param wait  窗口的间隔
* @param immediate 设置为true，是否立即调用函数	
* @return function  返回客户端函数
*/</span>
<span class="token keyword">function</span> <span class="token function">debounce</span><span class="token punctuation">(</span><span class="token parameter">func<span class="token punctuation">,</span>wait<span class="token operator">=</span><span class="token number">50</span><span class="token punctuation">,</span>immediate<span class="token operator">=</span><span class="token boolean">true</span></span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    
    <span class="token keyword">let</span> timer<span class="token punctuation">,</span>context<span class="token punctuation">,</span>args
    
    <span class="token keyword">const</span> <span class="token function-variable function">later</span><span class="token operator">=</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">=&gt;</span><span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">=&gt;</span><span class="token punctuation">{</span>
        
        <span class="token comment">// 延迟函数执行完毕，清空缓存的定时器序号</span>
        timer<span class="token operator">=</span><span class="token keyword">null</span>
        
        <span class="token comment">// 使用之前缓存的参数和上下文</span>
        <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token operator">!</span>immediate<span class="token punctuation">)</span><span class="token punctuation">{</span>
            <span class="token function">func</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span>context<span class="token punctuation">,</span>args<span class="token punctuation">)</span>
            context<span class="token operator">=</span>args<span class="token operator">=</span><span class="token keyword">null</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span><span class="token punctuation">,</span>wait<span class="token punctuation">)</span>
    
    <span class="token comment">// 这里返回的函数时每次实际调用的函数</span>
    
    <span class="token keyword">return</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>params</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token comment">// 没有创建延迟执行函数，later，创建一个</span>
        <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token operator">!</span>timer<span class="token punctuation">)</span><span class="token punctuation">{</span>
            timer<span class="token operator">=</span><span class="token function">later</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
            
            <span class="token comment">// 如果是立即执行，调用函数</span>
            
            <span class="token comment">// 定时器结束后执行，缓存上下文</span>
            <span class="token keyword">if</span><span class="token punctuation">(</span>immediate<span class="token punctuation">)</span><span class="token punctuation">{</span>
                func<span class="token punctuation">.</span><span class="token function">applt</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span>params<span class="token punctuation">)</span>
            <span class="token punctuation">}</span><span class="token keyword">else</span><span class="token punctuation">{</span>
                context<span class="token operator">=</span><span class="token keyword">this</span>
                args<span class="token operator">=</span>params
            <span class="token punctuation">}</span>
        <span class="token punctuation">}</span><span class="token keyword">else</span><span class="token punctuation">{</span>
            
            <span class="token comment">// 清除之前的定时器</span>
            <span class="token function">clearTimeout</span><span class="token punctuation">(</span>timer<span class="token punctuation">)</span>
            timer<span class="token operator">=</span><span class="token function">later</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
        <span class="token punctuation">}</span>
        
        
    <span class="token punctuation">}</span>
    
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br><span class="line-number">31</span><br><span class="line-number">32</span><br><span class="line-number">33</span><br><span class="line-number">34</span><br><span class="line-number">35</span><br><span class="line-number">36</span><br><span class="line-number">37</span><br><span class="line-number">38</span><br><span class="line-number">39</span><br><span class="line-number">40</span><br><span class="line-number">41</span><br><span class="line-number">42</span><br><span class="line-number">43</span><br><span class="line-number">44</span><br><span class="line-number">45</span><br><span class="line-number">46</span><br><span class="line-number">47</span><br><span class="line-number">48</span><br><span class="line-number">49</span><br></div></div><ul><li>对于按钮防点击来说的实现：如果函数是⽴即执⾏的，就⽴即调⽤，如果函数是延迟执⾏
的，就缓存上下⽂和参数，放到延迟函数中去执⾏。⼀旦我开始⼀个定时器，只要我定时
器还在，你每次点击我都重新计时。⼀旦你点累了，定时器时间到，定时器重置为
null ，就可以再次点击了</li></ul> <h4 id="节流"><a href="#节流" class="header-anchor">#</a> 节流</h4> <blockquote><p>防抖和节流的本质是不一样的，防抖动是将多次执行变成最后一次执行，节流是将多次执行变成每个一段时间执行</p></blockquote> <ul><li>简单版本</li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> timer<span class="token operator">=</span><span class="token keyword">null</span><span class="token punctuation">;</span><span class="token comment">//预定义一个节流阀</span>
<span class="token function">$</span><span class="token punctuation">(</span>document<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">on</span><span class="token punctuation">(</span><span class="token string">'mousemove'</span><span class="token punctuation">,</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">e</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token keyword">if</span><span class="token punctuation">(</span>timer<span class="token punctuation">)</span><span class="token punctuation">{</span><span class="token keyword">return</span><span class="token punctuation">}</span><span class="token comment">//判断节流阀是否为空</span>
timer<span class="token operator">=</span><span class="token function">setTimerout</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token function">$</span><span class="token punctuation">(</span>angel<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">css</span><span class="token punctuation">(</span><span class="token string">'left'</span><span class="token punctuation">,</span>e<span class="token punctuation">.</span>pageX<span class="token operator">+</span><span class="token string">'px'</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">css</span><span class="token punctuation">(</span><span class="token string">'top'</span><span class="token punctuation">,</span>e<span class="token punctuation">.</span>pageY<span class="token operator">+</span><span class="token string">'px'</span><span class="token punctuation">)</span>
timer<span class="token operator">=</span><span class="token keyword">null</span><span class="token punctuation">;</span><span class="token comment">//清空节流阀,方便下次开启延时器</span>


</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><h3 id="q5">6. 谈谈变量提升</h3> <blockquote><p>当执行JS代码时，会生成执行环境，只要代码不是写在函数中的，就是在全局执行环境中，函数中的代码会产生函数执行环境，只此两种执行环境</p></blockquote> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token function">b</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// call b</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span> <span class="token comment">// undefined</span>
<span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token string">'Hello world'</span>
<span class="token keyword">function</span> <span class="token function">b</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
 console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'call b'</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>

</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><blockquote><p>变量提升</p> <p>函数和变量提升的原因，将声明的代码移动到了顶部，准确的解释：在生成执行环境时，会有两个阶段，第一阶段是创建的阶段，JS解释器会找出需要提升的变量和函数，并且给它们提前在内存中开辟空间，函数的华会将整个函数存入内存中，变量只声明并赋值为undefined，第二个阶段，代码执行阶段，我们可以直接提前使用</p></blockquote> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token function">b</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// call b second</span>
<span class="token keyword">function</span> <span class="token function">b</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
 console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'call b fist'</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>
<span class="token keyword">function</span> <span class="token function">b</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
 console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'call b second'</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>
<span class="token keyword">var</span> b <span class="token operator">=</span> <span class="token string">'Hello world'</span>

</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><ul><li><p>相同的函数会覆盖上一个函数，并且函数优先于变量提升</p></li> <li><p>let关键字不支持变量提升</p></li></ul> <h3 id="q7">7. 什么是单线程，和异步的关系</h3> <ul><li>单线程：只有一个线程，只能做一件事</li> <li>原因：避免DOM渲染的冲突
<ul><li>浏览器需要渲染DOM</li> <li>js可以修改DOM结构</li> <li>js执行的时候，浏览器DOM渲染会暂停</li> <li>两段js也不能同时执行，都修改DOM就冲突了</li> <li>webworker支持多线程，但是不能访问DOM</li></ul></li> <li>解决方案：异步</li></ul> <h3 id="q12">12.实现效果，点击容器内的图标，图标边框变成border 1px solid red,点击空白处重置</h3> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">const</span> box <span class="token operator">=</span>document<span class="token punctuation">.</span><span class="token function">getElementById</span><span class="token punctuation">(</span><span class="token string">'box'</span><span class="token punctuation">)</span>
<span class="token keyword">function</span> <span class="token function">isIcon</span><span class="token punctuation">(</span><span class="token parameter">traget</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">return</span> target<span class="token punctuation">.</span>className<span class="token punctuation">.</span><span class="token function">includes</span><span class="token punctuation">(</span><span class="token string">'icon'</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>

box<span class="token punctuation">.</span><span class="token function-variable function">onClick</span><span class="token operator">=</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">e</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    e<span class="token punctuation">.</span><span class="token function">stopPropagation</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    <span class="token keyword">const</span> target<span class="token operator">=</span>e<span class="token punctuation">.</span>target
    
    <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token function">isIcon</span><span class="token punctuation">(</span>target<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        target<span class="token punctuation">.</span>style<span class="token punctuation">.</span>border<span class="token operator">=</span><span class="token string">'1px solid red'</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token keyword">const</span> doc<span class="token operator">=</span>document

doc<span class="token punctuation">.</span><span class="token function-variable function">onClick</span><span class="token operator">=</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">e</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    
    doc<span class="token punctuation">.</span><span class="token function-variable function">onClick</span><span class="token operator">=</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">e</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        
        <span class="token keyword">const</span> children<span class="token operator">=</span>box<span class="token punctuation">.</span>children
        
        <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">let</span> i<span class="token operator">=</span><span class="token number">0</span><span class="token punctuation">;</span>i<span class="token operator">&lt;</span>children<span class="token punctuation">.</span>length<span class="token punctuation">;</span>i<span class="token operator">++</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
            <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token function">isIcon</span><span class="token punctuation">(</span>children<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
                children<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">.</span>style<span class="token punctuation">.</span>border<span class="token operator">=</span><span class="token string">'none'</span>
            <span class="token punctuation">}</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br></div></div><h3 id="q13">13.简单实现双向数据绑定mvvm</h3> <div class="language-html line-numbers-mode"><pre class="language-html"><code><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>input</span> <span class="token attr-name">id</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>input<span class="token punctuation">&quot;</span></span> <span class="token punctuation">/&gt;</span></span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">const</span> data<span class="token operator">=</span><span class="token punctuation">{</span><span class="token punctuation">}</span>

<span class="token keyword">const</span> input<span class="token operator">=</span>document<span class="token punctuation">.</span><span class="token function">getElementById</span><span class="token punctuation">(</span><span class="token string">&quot;input&quot;</span><span class="token punctuation">)</span>

Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>data<span class="token punctuation">,</span><span class="token string">'text'</span><span class="token punctuation">,</span><span class="token punctuation">{</span>
    <span class="token function">set</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">{</span>
        input<span class="token punctuation">.</span>value<span class="token operator">=</span>value
        <span class="token keyword">this</span><span class="token punctuation">.</span>value<span class="token operator">=</span>value
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>

input<span class="token punctuation">.</span><span class="token function-variable function">onChange</span><span class="token operator">=</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">e</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    data<span class="token punctuation">.</span>text<span class="token operator">=</span>e<span class="token punctuation">.</span>target<span class="token punctuation">.</span>value
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><h3 id="q14">14.实现Storage,使得该对象为单例，并对localstorage进行封装设置值setItem(key,value)和getItem(key)</h3> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> instance<span class="token operator">=</span><span class="token keyword">null</span>

<span class="token keyword">class</span> <span class="token class-name">Strorage</span><span class="token punctuation">{</span>
    
    <span class="token keyword">static</span> <span class="token function">getInstance</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token operator">!</span>instance<span class="token punctuation">)</span><span class="token punctuation">{</span>
            instance<span class="token operator">=</span><span class="token keyword">new</span> <span class="token class-name">Storage</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
        <span class="token punctuation">}</span>
        
        <span class="token keyword">return</span> instance
    <span class="token punctuation">}</span>
    
    <span class="token function-variable function">setItem</span><span class="token operator">=</span><span class="token punctuation">(</span><span class="token parameter">key<span class="token punctuation">,</span>value</span><span class="token punctuation">)</span><span class="token operator">=&gt;</span>localStorage<span class="token punctuation">.</span><span class="token function">setItem</span><span class="token punctuation">(</span>key<span class="token punctuation">,</span>value<span class="token punctuation">)</span>

	<span class="token function-variable function">getItem</span><span class="token operator">=</span><span class="token parameter">key</span><span class="token operator">=&gt;</span>localStorage<span class="token punctuation">.</span><span class="token function">getItem</span><span class="token punctuation">(</span>key<span class="token punctuation">)</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><h3 id="q15">15.说说event loop</h3> <ul><li><p>js是单线程的，主要任务是处理用户的交互，用户的交互就是dom的增删改，使用事件队列的方式，一次事件循环处理一个事件响应，使得脚本执行相对连续，</p></li> <li><p>执行流程</p> <ul><li>一次事件循环中，手写执行主任务，然后回去查找是否有微任务，如果有那就有序执行微任务，没有，则查找宏任务进行执行</li></ul></li></ul> <h3 id="q16">16.说说事件流</h3> <ul><li>事件流分为两种，捕获事件流和冒泡事件流
<ul><li>捕获事件流从根节点开始执行，一直往子节点查找执行，直到查找目标节点</li> <li>冒泡事件流从目标节点开始执行，一直往父节点冒泡执行，直到根节点</li></ul></li> <li>事件流分为三个阶段
<ul><li>捕获节点</li> <li>处于目标节点阶段</li> <li>冒泡阶段</li></ul></li></ul> <h3 id="q19">19.手写实现promise</h3> <ul><li>由于Promise为状态机，先定义状态</li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> <span class="token constant">PENDING</span><span class="token operator">=</span><span class="token number">0</span> <span class="token comment">// 进行中</span>
<span class="token keyword">var</span> <span class="token constant">FULFILLED</span><span class="token operator">=</span><span class="token number">1</span> <span class="token comment">// 成功</span>
<span class="token keyword">var</span> REJEcted<span class="token operator">=</span><span class="token number">2</span> <span class="token comment">// 失败</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><ul><li>基本代码</li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">Promise</span><span class="token punctuation">(</span><span class="token parameter">fn</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">var</span> state<span class="token operator">=</span><span class="token constant">PENDING</span> <span class="token comment">// 存储状态Pending，fulfilled,rejected</span>
    <span class="token keyword">var</span> value<span class="token operator">=</span><span class="token keyword">null</span> <span class="token comment">// 存储成功或者失败的结果值</span>
    <span class="token keyword">var</span> handlers<span class="token operator">=</span><span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token comment">// 存储成功或者失败的处理程序</span>
    
    <span class="token keyword">function</span> <span class="token function">fulfilled</span><span class="token punctuation">(</span><span class="token parameter">result</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        state<span class="token operator">=</span><span class="token constant">FULFILLED</span>
        value<span class="token operator">=</span>result
        handlers<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span>handle<span class="token punctuation">)</span>
        handlers<span class="token operator">=</span><span class="token keyword">null</span>
    <span class="token punctuation">}</span>
    
    <span class="token comment">// 失败状态变化</span>
  <span class="token keyword">function</span> <span class="token function">reject</span><span class="token punctuation">(</span><span class="token parameter">error</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      state <span class="token operator">=</span> <span class="token constant">REJECTED</span><span class="token punctuation">;</span>
      value <span class="token operator">=</span> error<span class="token punctuation">;</span>
      handlers<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span>handle<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 处理函数，下文会提到</span>
      handlers <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
    
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br></div></div><ul><li>实现resolve方法</li></ul> <blockquote><p>resolve方法接受两种参数，一种为普通的值/对象，另外一种为一个Promise对象，如果是普通的值/对象，则直接把结果传递到下一个对象；
如果是一个 Promise 对象，则必须先等待这个子任务序列完成。</p></blockquote> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">Promise</span><span class="token punctuation">(</span><span class="token parameter">fn</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    
    <span class="token operator">...</span>
    
    <span class="token keyword">function</span> <span class="token function">resolve</span><span class="token punctuation">(</span><span class="token parameter">result</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        
        <span class="token keyword">try</span><span class="token punctuation">{</span>
            <span class="token keyword">var</span> then<span class="token operator">=</span><span class="token function">getThen</span><span class="token punctuation">(</span>result<span class="token punctuation">)</span>
            <span class="token keyword">if</span><span class="token punctuation">(</span>then<span class="token punctuation">)</span><span class="token punctuation">{</span>
                <span class="token function">doResolve</span><span class="token punctuation">(</span><span class="token function">then</span><span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span>result<span class="token punctuation">)</span><span class="token punctuation">,</span>resolve<span class="token punctuation">,</span>reject<span class="token punctuation">)</span>
                <span class="token keyword">return</span> 
            <span class="token punctuation">}</span>
            <span class="token function">fulfill</span><span class="token punctuation">(</span>result<span class="token punctuation">)</span>
        <span class="token punctuation">}</span><span class="token keyword">catch</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">{</span>
            
            <span class="token function">reject</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br></div></div><ul><li>resolve需要两个辅助方法getThen、doResolve</li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// getThen 检查如果value是一个Promise对象，则返回then方法等待执行完成</span>
<span class="token keyword">function</span> <span class="token function">getThen</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">var</span> t<span class="token operator">=</span><span class="token keyword">typeof</span> value<span class="token punctuation">;</span>
    
    <span class="token keyword">if</span><span class="token punctuation">(</span>value<span class="token operator">&amp;&amp;</span><span class="token punctuation">(</span>t<span class="token operator">===</span><span class="token string">'object'</span><span class="token operator">||</span>t<span class="token operator">===</span><span class="token string">'function'</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">var</span> then<span class="token operator">=</span>value<span class="token punctuation">.</span>then
        
        <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token keyword">typeof</span> then<span class="token operator">===</span><span class="token string">'function'</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
            <span class="token keyword">return</span> then
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
    
    <span class="token keyword">return</span> <span class="token keyword">null</span>
<span class="token punctuation">}</span>

<span class="token comment">// 异常检查函数，确保onFulfilled和onRejected两个函数只执行一个且只执行一次</span>

<span class="token keyword">function</span> <span class="token function">doResolve</span><span class="token punctuation">(</span><span class="token parameter">fn<span class="token punctuation">,</span>onFulfilled<span class="token punctuation">,</span>onRejected</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    
    <span class="token keyword">var</span> done<span class="token operator">=</span><span class="token boolean">false</span>
    
    <span class="token keyword">try</span><span class="token punctuation">{</span>
        <span class="token function">fn</span><span class="token punctuation">(</span>
            <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
            
            <span class="token keyword">if</span><span class="token punctuation">(</span>done<span class="token punctuation">)</span><span class="token punctuation">{</span>
                <span class="token keyword">return</span>
            <span class="token punctuation">}</span>
            
            done<span class="token operator">=</span><span class="token boolean">true</span>
            
            <span class="token function">onFulfilled</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span>
        <span class="token punctuation">}</span><span class="token punctuation">,</span>
            
            <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">reason</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        	<span class="token keyword">if</span> <span class="token punctuation">(</span>done<span class="token punctuation">)</span> <span class="token keyword">return</span><span class="token punctuation">;</span>
        	done <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
        	<span class="token function">onRejected</span><span class="token punctuation">(</span>reason<span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span>
        <span class="token punctuation">)</span>
        
    <span class="token punctuation">}</span><span class="token punctuation">.</span><span class="token function">catch</span><span class="token punctuation">(</span>ex<span class="token punctuation">)</span><span class="token punctuation">{</span>
        
        <span class="token keyword">if</span> <span class="token punctuation">(</span>done<span class="token punctuation">)</span> <span class="token keyword">return</span><span class="token punctuation">;</span>
    	done <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
    	<span class="token function">onRejected</span><span class="token punctuation">(</span>ex<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br><span class="line-number">31</span><br><span class="line-number">32</span><br><span class="line-number">33</span><br><span class="line-number">34</span><br><span class="line-number">35</span><br><span class="line-number">36</span><br><span class="line-number">37</span><br><span class="line-number">38</span><br><span class="line-number">39</span><br><span class="line-number">40</span><br><span class="line-number">41</span><br><span class="line-number">42</span><br><span class="line-number">43</span><br><span class="line-number">44</span><br><span class="line-number">45</span><br><span class="line-number">46</span><br><span class="line-number">47</span><br><span class="line-number">48</span><br></div></div><ul><li>上面已经完成了一个完整的内部状态机，但我们并没有暴露一个方法解析或者观察Promise，我们开始解析Promise</li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">Promise</span><span class="token punctuation">(</span><span class="token parameter">fn</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    
    <span class="token operator">...</span>
    <span class="token function">doResolve</span><span class="token punctuation">(</span>fn<span class="token punctuation">,</span>resolve<span class="token punctuation">,</span>reject<span class="token punctuation">)</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><h3 id="then方法实现"><a href="#then方法实现" class="header-anchor">#</a> then方法实现</h3> <p>在实现<code>then</code>方法之前，我们这里实现了一个执行方法done，该方法用来处理执行then方法的回调函数，一下为promise.done(onFullfilled, onRejected)方法的几个点。</p> <ul><li>onFulfilled 和 onRejected 两者只能有一个被执行，且执行次数为一</li> <li>该方法仅能被调用一次, 一旦调用了该方法，则 promise 链式调用结束</li> <li>无论是否 promise 已经被解析，都可以调用该方法</li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">Promise</span><span class="token punctuation">(</span><span class="token parameter">fn</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token operator">...</span>
  <span class="token comment">// 不同状态，进行不同的处理</span>
  <span class="token keyword">function</span> <span class="token function">handle</span><span class="token punctuation">(</span><span class="token parameter">handler</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>state <span class="token operator">===</span> <span class="token constant">PENDING</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      handlers<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
      <span class="token keyword">if</span> <span class="token punctuation">(</span>state <span class="token operator">===</span> <span class="token constant">FULFILLED</span> <span class="token operator">&amp;&amp;</span> <span class="token keyword">typeof</span> handler<span class="token punctuation">.</span>onFulfilled <span class="token operator">===</span> <span class="token string">'function'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        handler<span class="token punctuation">.</span><span class="token function">onFulfilled</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span>
      <span class="token keyword">if</span> <span class="token punctuation">(</span>state <span class="token operator">===</span> <span class="token constant">REJECTED</span> <span class="token operator">&amp;&amp;</span> <span class="token keyword">typeof</span> handler<span class="token punctuation">.</span>onRejected <span class="token operator">===</span> <span class="token string">'function'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        handler<span class="token punctuation">.</span><span class="token function">onRejected</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span>

  <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function-variable function">done</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">onFulfilled<span class="token punctuation">,</span> onRejected</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment">// 保证异步</span>
    <span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token function">handle</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token literal-property property">onFulfilled</span><span class="token operator">:</span> onFulfilled<span class="token punctuation">,</span> <span class="token literal-property property">onRejected</span><span class="token operator">:</span> onRejected<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br></div></div><p>当 Promise 被 resolved 或者 rejected 时，我们保证 handlers 将被通知。
then方法</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">Promise</span><span class="token punctuation">(</span><span class="token parameter">fn</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token operator">...</span>
  <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function-variable function">then</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">onFulfilled<span class="token punctuation">,</span> onRejected</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">var</span> self <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">Promise</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">resolve<span class="token punctuation">,</span> reject</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      self<span class="token punctuation">.</span><span class="token function">done</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">result</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> onFulfilled <span class="token operator">===</span> <span class="token string">'function'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
          <span class="token keyword">try</span> <span class="token punctuation">{</span>
            <span class="token comment">// onFulfilled方法要有返回值！</span>
            <span class="token keyword">return</span> <span class="token function">resolve</span><span class="token punctuation">(</span><span class="token function">onFulfilled</span><span class="token punctuation">(</span>result<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
          <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>ex<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">return</span> <span class="token function">reject</span><span class="token punctuation">(</span>ex<span class="token punctuation">)</span><span class="token punctuation">;</span>
          <span class="token punctuation">}</span>
        <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
          <span class="token keyword">return</span> <span class="token function">resolve</span><span class="token punctuation">(</span>result<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
      <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">error</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> onRejected <span class="token operator">===</span> <span class="token string">'function'</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
          <span class="token keyword">try</span> <span class="token punctuation">{</span>
            <span class="token keyword">return</span> <span class="token function">resolve</span><span class="token punctuation">(</span><span class="token function">onRejected</span><span class="token punctuation">(</span>error<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
          <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span>ex<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">return</span> <span class="token function">reject</span><span class="token punctuation">(</span>ex<span class="token punctuation">)</span><span class="token punctuation">;</span>
          <span class="token punctuation">}</span>
        <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
          <span class="token keyword">return</span> <span class="token function">reject</span><span class="token punctuation">(</span>error<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
      <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br></div></div><p>catch方法，我们直接调用then处理异常</p> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function-variable function">catch</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">errorHandle</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">,</span> errorHandle<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
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